The present invention relates to a process for the production of an improved laminated material. The invention relates in particular to a process for the production of a laminated material in which a film of cellulose acetate, in particular cellulose diacetate, is laminated to a printed substrate.
It is known to laminate clear glossy, matt or semi-matt films of materials such as cellulose acetate or diacetate to printed material, for use for example for book covers and in packaging material.
A range of cellulose diacetate films has recently been developed, which films have a matt or semi-matt finish which is particularly distinctive and attractive and which also has other desirable properties, which are unique to these materials. These films are plastics films, in particular cellulose diacetate films, which are marketed under the Trade Mark Semitone by Acordis Acetate Products Ltd (Clarifoil). Semi-matt or matt plastics films of this type, and a process for their manufacture, are described in European Patent Application No. 0 802 069 A1, the contents of which are incorporated herein by reference, and the films described in that patent application are particularly suitable for use in the material and process of the present invention. These matt or semi-matt films are characterised by a regular embossed pattern of depth 0.5 to 10 microns and repeat distance 20 to 400 microns and it is taught in the specification that these films can be laminated over printed paperboard. The characteristic properties of these films include the ability to emit a characteristic noise of reproducible pitch when two films are rubbed together and also a high resistance to finger marking. These properties are due to the embossed surface of the film and it has therefore up until now been the practice to use a wet, cold lamination process to laminate these films to a substrate using a lamination technique which does not involve heating the film, so that no deformation of the embossed surface of the film, and hence impairment of these characteristic properties, would occur in the lamination process.
Thermal lamination is a very popular process for book cover lamination, especially for paperback books. In a thermal lamination process, a melt coating is extruded onto the film, the coated film is chilled and then wound up as a roll of film with dry adhesive already applied. The laminator then needs to run this film over a simple hot roller, melting the adhesive, and pressure nip it onto the substrate, suitably a printed board, by running the film and substrate between nip rollers. The process is more expensive than wet lamination, but requires less skill, and therefore has been taken up by printers, who can therefore shorten their turn round times by not needing out-of-house contract lamination.
In an alternative form of thermal lamination, which is also referred to as encapsulation, a printed substrate such as a restaurant menu or a poster, is laminated between two sheets of film to encapsulate the paper. Encapsulation is generally carried out with thicker films and thicker coatings than are used for thermal lamination on one side of a substrate.
Matt finishes feature prominently for book cover lamination. Matt oriented polypropylene film is widely used, but is prone to scuffing and marking. In the United States, in particular, higher levels of humidity, in particular in southern states, mean that curl of book covers can be a severe problem. So called xe2x80x9clayflatxe2x80x9d nylon films have been introduced which give improved curl correction, but at the expense of even worse scratch and scuff resistance.
It is an object of the present invention to provide a process for the manufacture of an improved laminate. It is a further object of the present invention to provide a process for the manufacture of an improved laminate in which the disadvantages of known laminates are reduced or substantially obviated.
The present invention provides a process for the manufacture of a laminate which comprises a thermoplastic plastics film having, on at least one side, an embossed pattern of protrusions having a height in the range of 0.5 to 10 microns and a separation in the range 20 to 400 microns, laminated to a substrate in which the improvement comprises the use of a thermal lamination process, which process includes the steps of applying a melt coating between the film and the substrate and running the film and substrate, whilst the adhesive is in a heated condition, between nip rollers to apply pressure thereto, so as to effect lamination of the film to the substrate. In a preferred embodiment of the process according to the present invention, the thermoplastic plastics film is a matt or semi-matt plastics film and is preferably a cellulose acetate film, more preferably a cellulose diacetate film.
The melt coating is preferably applied to the film, more preferably by an extrusion process. In a particularly preferred embodiment of the process according to the invention, the coated film is chilled after the adhesive has been applied and the coated film is subsequently run over a heated roller to melt the adhesive.
In a preferred embodiment of the process according to the invention, the process further includes the step of winding up the roll of film with dry adhesive applied, between the chilling step and the heating step. In a particularly preferred embodiment of the process according to the invention, the dry melt adhesive is a polyethylene homopolymer or an ethylene vinyl acetate copolymer which has a melting point between 140 and 180xc2x0 C. and is preferably applied by extrusion coating at a temperature close to the melting point, at which temperature the polyethylene homopolymer or ethylene vinyl acetate copolymer is molten.
The matt or semi-matt thermoplastic plastics film for use in the process according to the invention preferably has a thickness of approximately 15xcexc and the adhesive coating is preferably applied at a thickness of 17 to 18xcexc, at a temperature significantly higher than the softening point of the film.
The coated film is run over a heated roller at a temperature of about 115xc2x0 C., with the side of the film with the protrusions in contact with the roller, and the adhesive is heated to a temperature of about 100xc2x0 C. or above, at which temperature the adhesive is softened.
The acetate film is preferably treated prior to application of the adhesive, in a manner known per se for conventional lamination films, including acetate films, with a primer and/or by a corona discharge treatment, in order to improve the adhesion between the acetate and the adhesive and between the film and the substrate.
Where the acetate film is treated with a primer prior to the application of adhesive, the primer is preferably one conventionally used for coating, such as a polyurethane or isocyanate based primer in either aqueous solution or in an organic solvent system. Typical coating thicknesses are less than one micron.
Where the acetate film is treated by a corona discharge treatment prior to application of the adhesive, the corona is generated by applying a high voltage signal to an electrode. The substrate to be treated is passed below the electrode; beneath the substrate is a dielectric layer, with an earthed plane below the dielectric layer. When a sufficiently high voltage is applied, electrical breakdown occurs in the air gap between the electrode and the substrate, so that the substrate (film) surface is bombarded with high energy electrons. The net effect is to raise the surface energy of the film substrate. The peak voltage applied is preferably up to 30 kV, preferably at an ac frequency of between 9 to 50 kHz.
It has surprisingly been found that when a matt or semi-matt thermoplastic plastics film having an embossed pattern of protrusions having a height in the range of 0.5 to 10 microns and a separation in the range 20 to 400 microns is laminated to a substrate by means of a thermal lamination process, the resultant laminated material retains the characteristic properties given to the film by the protrusions in the unlaminated material.
This result is particularly surprising in view of the fact that uncoated cellulose diacetate film demonstrates significant softening in the temperature range 140 to 150xc2x0 C. and even at temperatures above 100xc2x0 C., there is noticeable shrinkage in both width and length.
In particular, the laminates according to the invention demonstrate substantially better resistance to finger marking, scratching, scuffing and abrasion than laminates of matt oriented polypropylene or nylon. The print sharpness and contrast, and the colour of the laminated material are enhanced and there is an improvement in the angle of viewing, partially but not entirely due to gloss reduction. It has also been found that the laminates, when applied over printed text, improve the readability of the text, especially where the reader is dyslexic. It is believed that this effect is due to increased focussing of the reader""s eye as the text is being scanned.
It has also been found that the coated film produced in the process according to the invention demonstrates increased tear resistance compared to the uncoated film, both in the initiation and the propagation of tears. This increased tear resistance is of value both during the lamination process, where it is possible to carry out the process more rapidly, and for the finished article, which demonstrates increased resistance to tearing.
In addition, cellulose diacetate films tend to expand and contract with changing humidity in the same way as paper, although paper is more directional because of the effect of the fibre grain. In principle therefore, as humidity changes, an acetate film will tend to move in the same direction as the paper to which it is laminated, helping to reduce curl effects. The laminates according to the present invention tend to give similar, or slightly improved results compared to matt oriented polypropylene film. Matt nylon indeed gives the best layflat properties, but the improvement over acetate is not significant.
Finally, the laminates according to the present invention, in particular those which comprise a film of the type described and claimed in European Patent Application No. 0 802 069 A1 and marketed under the Trade Mark xe2x80x9cSemitonexe2x80x9d have a very distinctive xe2x80x9chandlexe2x80x9d compared with the known laminates. Matt oriented polypropylene film is waxy; matt nylon is rather rough and matt polyethylene terepthalate has a xe2x80x9csyntheticxe2x80x9d feel and also curls and fingermarks extremely badly, whereas the laminates according to the present invention have a softer more natural feel.